Aden, Yemen — Across the vast, rugged terrain of southern Yemen, an escalating campaign of U.S. drone strikes is stirring increasing sympathy for al-Qaeda-linked militants and driving tribesmen to join a network linked to terrorist plots against the United States.

After recent U.S. missile strikes, mostly from unmanned aircraft, the Yemeni government and the United States have reported that the attacks killed only suspected al-Qaeda members. But civilians have also died in the attacks, said tribal leaders, victims’ relatives and human rights activists.

“These attacks are making people say, ‘We believe now that al-Qaeda is on the right side,’ ” said businessman Salim al-Barakani, adding that his two brothers — one a teacher, the other a cellphone repairman — were killed in a U.S. strike in March.

Since January, as many as 21 missile attacks have targeted suspected al-Qaeda operatives in southern Yemen, reflecting a sharp shift in a secret war carried out by the CIA and the Joint Special Operations Command that had focused on Pakistan.

But as in the tribal areas of Pakistan, where U.S. drone strikes have significantly weakened al-Qaeda’s capabilities, an unintended consequence of the attacks has been a marked radicalization of the local population.

U.S. drone strikes on rise in Yemen (The Washington Post/The Long War Journal)

The evidence of radicalization emerged in more than 20 interviews with tribal leaders, victims’ relatives, human rights activists and officials from four provinces in southern Yemen where U.S. strikes have targeted suspected militants. They described a strong shift in sentiment toward militants affiliated with the transnational network’s most active wing, al-Qaeda in the ­Arabian Peninsula, or AQAP.

“The drone strikes have not helped either the United States or Yemen,” said Sultan al-Barakani, who was a top adviser to former president Ali Abdullah Saleh. “Yemen is paying a heavy price, losing its sons. But the Americans are not paying the same price.”

In 2009, when President Obama was first known to have authorized a missile strike on Yemen, U.S. officials said there were no more than 300 core AQAP members. That number has grown in recent years to 700 or more, Yemeni officials and tribal leaders say. In addition, hundreds of tribesmen have joined AQAP in the fight against the U.S.-backed Yemeni government.

As AQAP’s numbers and capabilities have grown, so has its reach and determination. That was reflected in a suicide bombing last week in the capital, Sanaa, that killed more than 100 people, mostly Yemeni soldiers.

On their Web sites, on their Facebook pages and in their videos, militants who had been focused on their fight against the Yemeni government now portray the war in the south as a jihad against the United States, which could attract more recruits and financing from across the Muslim world. Yemeni tribal Web sites are filled with al-Qaeda propaganda, including some that brag about killing Americans.

“Every time the American attacks increase, they increase the rage of the Yemeni people, especially in al-Qaeda-controlled areas,” said Mohammed al-Ahmadi, legal coordinator for Karama, a local human rights group. “The drones are killing al-Qaeda leaders, but they are also turning them into heroes.”

An escalated campaign

Obama’s top counterterrorism adviser, John O. Brennan, has publicly defended the use of drone strikes, arguing that their precision allows the United States to limit civilian casualties and lessen risks for U.S. military personnel. The decision to fire a missile from a drone, he said, is taken with “extraordinary care and thoughtfulness.”

National Security Council spokesman Tommy Vietor said the administration’s counter­terrorism strategy in Yemen is “guided by the view that we must do what is necessary to disrupt AQAP plots against U.S. interests” and to help the Yemeni government build up its capabilities to fight AQAP.

“While AQAP has grown in strength over the last year, many of its supporters are tribal militants or part-time supporters who collaborate with AQAP for self-serving, personal interests rather than affinity with al-Qaeda’s global ideology,” Vietor said. “The portion of hard-core, committed AQAP members is relatively small.”

The dramatic escalation in drone strikes in Yemen followed foiled plots by AQAP to bomb a U.S. airliner headed to Detroit in 2009 and to send parcel bombs via cargo planes to Chicago the following year. In April, Saudi intelligence agents helped foil an AQAP plot to plant a suicide bomber on a U.S.-bound plane.

On May 6, a U.S. drone strike killed Fahd al-Quso, a senior al-Qaeda leader who was on the FBI’s most-wanted list for his role in the 2000 bombing of the USS Cole in Aden, an attack that killed 17 American sailors. The drone strike in Shabwa province also killed a second man, whom U.S. and Yemeni officials described as another al-Qaeda militant.

But according to his relatives, the man was a 19-year-old named Nasser Salim who was tending to his farm when Quso arrived in his vehicle. Quso knew Salim’s family and was greeting him when the missiles landed.

“He was torn to pieces,” said Salim’s uncle, Abu Baker Aidaroos, 30, a Yemeni soldier. “He was not part of al-Qaeda. But by America’s standards, just because he knew Fahd al-Quso, he deserved to die with him.”

Out of anger, Aidaroos said, he left his unit in Abyan province, the nexus of the fight against the militants. Today, instead of fighting al-Qaeda, he sympathizes with the group — not out of support for its ideology, he insists, but out of hatred for the United States.

‘More hostility’ toward U.S.

The U.S. strikes, tribal leaders and Yemeni officials say, are also angering powerful tribes that could prevent AQAP from gaining strength. The group has seized control of large swaths of southern Yemen in the past year, while the government has had to counter growing perceptions that it is no more than an American puppet.

“There is more hostility against America because the attacks have not stopped al-Qaeda, but rather they have expanded, and the tribes feel this is a violation of the country’s sovereignty,” said Anssaf Ali Mayo, Aden head of al-Islah, Yemen’s most influential Islamist party, which is now part of the coalition government. “There is a psychological acceptance of al-Qaeda because of the U.S. strikes.”

Quso and Salim are from the Awlak tribe, one of the most influential in southern Yemen. So was Anwar al-Awlaki, the Yemeni American preacher who was thought to be a senior AQAP leader and was killed in September by a U.S. strike. The following month, another U.S. strike killed Awlaki’s 16-year-old son, Abdulrahman, also an American citizen, generating outrage across Yemen.

Awlak tribesmen are businessmen, lawmakers and politicians. But the strikes have pushed more of them to join the militants or to provide AQAP with safe haven in their areas, said tribal leaders and Yemeni officials.

“The Americans are targeting the sons of the Awlak,” Aidaroos said. “I would fight even the devil to exact revenge for my nephew.”

In early March, U.S. missiles struck in Bayda province, 100 miles south of Sanaa, killing at least 30 suspected militants, according to Yemeni security officials. But in interviews, human rights activists and victims’ relatives said many of the dead were civilians, not fighters.

Villagers were too afraid to go to the area. Al-Qaeda militants took advantage and offered to bury the villagers’ relatives. “That made people even more grateful and appreciative of al-Qaeda,” said Barakani, the businessman. “Afterwards, al-Qaeda told the people, ‘We will take revenge on your behalf.’ ”

In asserting responsibility for last week’s bombing in Sanaa, Ansar al-Sharia — the name by which AQAP goes in southern Yemen — declared that the attack was revenge for what it called the U.S. war on its followers.

The previous week, al-Qaeda’s supreme leader, Ayman al-Zawahiri, released a video portraying Yemeni President Abed Rabbo Mansour Hadi, who took office in February and vowed to fight AQAP, as an “agent” of the United States.

In some cases, U.S. strikes have forced civilians to flee their homes and have destroyed homes and farmland. Balweed Muhammed Nasser Awad, 57, said he and his family fled the city of Jaar last summer after his son, a fisherman, was killed in a U.S. strike targeting suspected al-Qaeda militants. Today, they live in a classroom in an Aden school, along with hundreds of other refugees from the conflict.

“Ansar al-Sharia had nothing to do with my son’s death. He was killed by the Americans,” Awad said. “He had nothing to do with terrorism. Why him?”

No Yemeni has forgotten the U.S. cruise missile strike in the remote tribal region of al-Majala on Dec. 17, 2009 — the Obama administration’s first known missile strike inside Yemen. The attack killed dozens, including 14 women and 21 children, and whipped up rage at the United States.

Today, the area is a haven for militants, said Abdelaziz Muhammed Hamza, head of the Revolutionary Council in Abyan province, a group that is fighting AQAP. “All the residents of the area have joined al-Qaeda,” he said.

Sudarsan Raghavan is The Washington Post’s Cairo bureau chief and has reported from more than 60 countries. He has been posted in Baghdad, Kabul, Johannesburg, Madrid and Nairobi. Raghavan has covered the wars in Iraq and Afghanistan, the Israeli-Palestinian conflict, and the 2011 Arab revolutions, as well as reported from 17 African wars.